Cathode ray apparatus



Oct. 22, 1940. D J MYNALL' 2,219,193

cu'uom: RAY APPARATUS Filed April 21, 1938 2 Sheets- Sheet 1 Fig.1. Fig. 2.

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e35 and which approximates asquare root relation Patented Oct. 22, 1940 CATHODE RAYAIIPAIRATYUS Dennis James Mynall, Rugby, England, assignor to General Electric Company, a corporation of New York Application April 21, 1938, Serial Nb. 203,311

In Great Britain May'l, 1937 "3Clain 1s. (0 1150%???) i I electromagnetic means for focusing the ray in I I whichjthe focusing meansis so arranged that the inventicnrelates to the operation of cathode raytubes with magnetic focusing and is par-,

ticularly though not exclusively concerned with r the: use of such arrangements in television reception. V Where a cathode'ray tube is focused by means of a magnetic lens, and the electrical supplies to the tube anode-cathode circuit and the magnetic lens are taken from a common source of alternating or direct current, it is found that a variation of focus is consequent upon a, variation of mains voltage, resulting in the'necessity 'for; manual control to counteract the. changes. -This, is :an undesirable feature in a television receiver-and it is the object of this invention to provide a simple means of automatic compensa- ,gtion-over a rangesufiicient to cover the variations of voltage encountered in practice.

-,-In,orcler that this result may obtain it is necessary" (i; e. theidealcondition) that the flux produced by the magnetic means shall vary, as the square root of the voltageof' the energizing source (it being assumed that the electrostatic field betweenthe cathode and the accelerating anode varies directly as such voltage). With the ordinary magnetic system operating in an unsat- --,urated state this resultvwill not be obtained, since both magnetizingrcurrent and flux increase approximately linearly with the exciting voltage. However, it is foundthatv if means are provided for causing the magnetic system to operate on various embo diments of the invention.

' Fig. 1 shows the difference between theideal 'the'knee of the saturation curve, the desired conditionwill be fulfilled. That is to say, the magnetic fiuxwill increase as a" function which is l'es's thanza linear function of the exciting voltage thereto. q

The features of the invention which'I' desire to protect herein arepointed out with particularity in theappended claims. The invention itself, to- "gether with further objects and advantages thereof may best be understood by reference to the "following description taken in connection with the drawings in which Figs. 1 through 4 are curves which are referred 'to' in the explanation o'f'the invention, and Figs.'.5 through 8 illustrate relationship of magnetizing current and anode- 'c'a'thode voltage and'the" relationship which is 'attainedw'ith the energizing arrangements heretofore employed. The point of intersectionP- represents acondition of focus andis taken as the best settingfor "the average working of"V. It is evident from'F'ig. 1 that a variation 'of V will result in a corresponding change in I which is more than that ideally required. This is why manual readjustment is necessary; 5

The preferred embodimentvofthe present invention consists in a cathode ray tube having curve showing thee-relation which must hold between I (the current fiowing through the focus- .ing means) for varying values of V (the anode-4 cathode voltage of the tube) in order to maintain the ray'in focus, and the curve showing the relaition between I and V which holds under working conditions are so closely aligned over a predetermined range of the value of V that the ray is substantially maintained in focus overthe said, range.

According to a feature of the present invention, the alignmentof the two curves is effected over a finite range of operating voltage by designing the magnetic circuitof the lens so that it operates in the condition of saturation. The ideal relationship between I and V may be written J=V s but inthis cases. is constant only for the lower values of Land increases with I in the condition of saturation. Thus the effect of saturation is to give an S-shaped=ideal curveto which the practical curve (of the form I:bV) may be aligned suii'iciently closely to provide a range of values of V over which the focus does not appreciably change. Thisis shown in Fig. 2. The magnetic saturation may, for example, be introduced by a suitable restriction intheiron circuit of the lens, and the relation i I=aV controlledyif required, by shunting the restriction magnetically. The control of the relation I=bV maybeobtainecl in the normal manner by resistance in shunt and/or series" with'the resistance of the lens winding.

With a given formof :lens construction the compensation is obtained only at the expense of increased, energy dissipation in the lens. This may be undesirable, and accordingly a further feature of the invention consists in the introduction .of 1 an auxiliary magnetic field which is independent of the voltage V, working in con- J'unction withthe normal field.,. The addition of the constant'fielcl causes the IV relationship requiredfor constant focus to, take the form "I: ZVc, where c is constant, a is as before, and I now represents the current producingv the Variable part of the field. Fig.3 illustrates the superposed constant field case wherethere is no sat uration, andFig. 4 gives the corresponding case where .saturation occurs, f It will beseen" that either curve can bematched over aworkingjrange of v; ,Tn' eo stanp field,.on which the value of cdepends, is preferably obtained by incorporating a permanentmagnet in the same system as that which provides the variablefield, initial'control being exercised, if required, by means of a mag! netic shunt or shunts.

The constant field may also be obtained by the use of a permanent magnet physically separate from theremainder of the system, by means of an auxiliary winding supplied from a constant current or constant voltage device, or by means of a separate lens with windings supplied from a constant current or constant voltage device.

Magnetic circuits suitable for carrying out the invention are illustrated by way of example, in the accompanying drawings, the figures of which have been numbered 5 to I consecutively with those of the drawings hereinbefore referredto.

In Fig. 5 an electromagnetic electron-optical lens is shown in which a winding A in the annular tunnel B sends flux through the magnetic body of the lens (which may, for example, be of mild steel) and across the annular air gap C. The leakage flux from the air gap is used to focus a beam of electrons travelling in the general direction XXi. As so far described, the lens construction is well-known, but the drawings show a restriction D in the magnetic circuit which can be shunted by the adjustable element, E, made of similar material to the body of the lens. This element shunts the restriction, so making it effectively of greater area, so that magnetic saturation sets in at a higher total flux in the magnetic circuit. This allows the saturation effect to be adjusted to function correctly over a fairly wide range of working conditions. If normal working conditions are not subject to extensive change, it is practicable to dispense with the adjustable element.

Fig. 6 shows a similar lens in which the restriction is situated at F. Adjustment may be effected by turning the magnetic shunt G, (made of similar material to the main body of the lens) in the threaded nonmagnetic ring H which is fixed to the main body. The effect of G is'to shunt flux from the saturable part F, so modifying the saturation curve of the magnetic circuit considered as a whole.

In extreme cases, where great fineness of control is required, it is possible to use the methods of Fig. 5 and Fig. 6 in combination on the same lens. The usefulness of this is that the two methods modify the normal saturation curve in rather I different manners, so that, when used together,

the variety of characteristics which can be com- -manded is greater than .for either alone.

Fig. 7 shows. a normal lens having a magnetic circuit J, to which has been afiiXed a ring shaped permanent magnet K which sends flux across the annular air gapL, via a part M of the magnetic circuit of the lens, and an axially adjustable annular pole piece, N. The pole piece N may be of .material similar to the magnetic circuit. The magnetic fluxes from the windings of J andfrom K are in such a sense that M carries their difference, which is never large. The adjustable element N allows the focusing field due to the magnet K to be set to the required amount, in relation to the focusing field from the other air gapO.

As before, it may be possible to dispense with the adjustments, if the working conditions are always to .be within known limits.

In Fig. 8 of the drawings I have illustrated the applicationof my invention in schematic form. Thus, there is shown a cathode ray tube IE! of conventional type having a heated cathode I l, a control grid 12, andan accelerating anode l3. The remaining electrodes of the device, including apair of feeder lines !5,

the deflecting electrodes, are not shown since they .are not relevant to my presentinvention.

Potential is supplied to the various electrodes from a source which is illustrated as comprising I6. These are connected across a potentiometer I! from which voltages of various values may be obtained. In this connection it will be noted that the anode I3 is connected to the potentiometer at a point l8 which is highly positive with respect to the cathode ll.

Around the shaft of the cathode ray tube there is provided a magnetic flux system comprising a magnetic core 20 of the character illustrated more completely in Fig. 5 and an energizing coil 2|. The terminals of the coil 2| are connected to the potentiometer l! at taps 23 and 24. In this way assurance is obtained that potential variations of the common source [5, l6 tending to affect'the velocity of the electron stream will produce simultaneous changes in the excitation of the coil?! so asto obtain constant focusing as described above.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a cathode ray tubehaving of the'voltage of the said source, whereby the focus of the beam remains substantially constant irrespective of changes in such voltage.

2. In combination, a cathode ray tube having means for developing an electron beam, means for producing an electrostatic field effective to accelerate the beam to a desired velocity, means including a magnetic structure for producing a magnetic field which coacts with the said electrostatic field to cause the beam to focus in a predetermined plane, a common voltage source for energizing both field-producing means, and means forming a part of the flux path of the magneticstructure for causingthe same to be sufiiciently saturated so that the field which is produced by such means varies approximately as the square root of the voltage of said source, whereby the beam focus remains substantially constant irrespective of changes in suchvoltage.

3. A cathode ray apparatus having means for developing an electron beam, means for producingan electrostatic field effective to accelerate the beam to a desired velocity, means for producing a magnetic field which coacts with the said electrostatic field to cause the beam to focus in a predetermined plane, and a common voltage source for energizing both'field-producing means, wherein the magnetic field-producing means includes a first magneticstructure producing a field of constant intensity and a second magnetic structure producing a field'of intensity which varies with the voltage of the said source, the two fields acting differentially and the relation between them being such that the resultant field varies approximately as the square root of the voltage source inthe normal operating range of the apparatus.

.DENNIS JAMES MYNALL. 

